Sealed enclosures filled with fill fluid are needed to protect measurement devices, such as pressure transmitters or the sensors in vortex-shedding meters, from aggressive environments. See, for example, the transmitter shown and described in U.S. Pat. No. 3,948,098 to Richardson et al. As described in the Richardson et al. patent, these devices typically include process isolation diaphragms which fluctuate in response to alternating pressure variations generated by a shedder placed in a stream of process fluid. Pressure applied to the diaphragms is propagated to a pressure sensing device such as a piezoelectric bimorph device sealed and protected from the process fluid via an electrically non-conductive hydraulic fill fluid. The frequency of pressure detected by the sensor is then measured to determine the mean flow velocity.
For such sensing devices, the measurement accuracy is critically dependant on the ability of a mechanically compliant structure to transmit pressure while, at the same time, protecting the sensing device from the aggressive process fluids. If fill fluid becomes contaminated, which can result from a breach of the isolation diaphragm, the accuracy of the measurements of the sensor will be significantly affected. Relying on these false readings can result in drastic consequences for a measured process.